Amplifier



M. MORRISON Feb. 9, 1954 AMPLIFIER Original Filed April 26, 1949 2 Sheets-Sheet 1 1mm TOR,

M. MORRISON Feb. 9, 1954 AMPLIFIER 2 Sheets-Sheet 2 Original Filed April 26, 1949 m E U. h lirmum... u 5 unooa ma on.

INVENTOR.

Patented Feb. 9, 1954 AMPLIFIER Montford Morrison, Upper Montclair, N. J.

Original application April 26, 1949, Serial No.

89,605, now Patent No. 2,623,944, dated December 30, 1952.

Divided and this application March 31, 1950, Serial No. 153,239

3 Claims.

This application is a required. division of application 89,605, filed April 26, 1949, now issued December 30, 1952, as Patent No. 2,623,944.

The present invention relates to electric wave filters and to the joint structure resulting from combining electric Wave filters with electronic tube amplifier component organizations.

Among the objects of the invention is to provide a band-pass filter which is, much smaller, much lighter, much less expensive to build, than prior art structures, and which makes practical the employment of a multiplicity of carriercurrent filter devices in small aircraft and in portable field apparatus. The smallest equivalent prior art filter known to the applicant measures 19" x x 3%" and weighs pounds. A filter equivalent in performance, constructed in accordance with the present invention, measures 5 x 2% x 3 and weighs 11 ounces.

Another object of the invention is to provide electronic tube amplifier organizations for joint use with filters of this invention, which are much smaller, much lighter, much less expensive to build, and which contribute jointly to the practicability of carrier-current devices in small aircraft and for portable field apparatus. Prior art amplifiers of equivalent performance, are usually more bulky than the filter, but weigh somewhat less. The present amplifier (without power supply) measures 2%" x 2% x 1%" (without output transformer) and the output transformer measures 1%" X 1%. The filter proper weighs 7 ounces and the output transformer weighs 6 ounces,

Such a filter, amplifier and output transformer combination, with 1 milliwatt input into the filter provides more than 1 watt in the secondary of the output transformer, with a single 6F8 tube operating within its published rating, and considerably greater outputs are obtained with plate voltages in excess of this rating or with tubes of higher ratings.

The complete channel organization of the 3 units described, weighs 1 /2 pounds. Greater output energy is obtained, with the same organization, with greater watt inputs.

Such results are clearly new and obviously unexpected in view of the prior art.

A further object of the invention is to provide a filter having the absolute minimum number of components for the performance provided.

A further object to provide a full two stage filter having a single high Q reactor tank circuit.

A further object is to provide a filter organiza 2 tion, in which energy is fed into the filter at both the input end and at the output end thereof.

A further object is to provide means to feed energy into the output end of a filter at the same time that energy is being fed into the input end thereof, thereby raising the overall Q of the filter, which results in reduction of size, weight and cost, and an increase in sharpness of definition in filter characteristics.

A further object is to provide means to limit or stabilize the above said energy fed into the output end of the filter, to an amount less than the overall losses in the filter, which amount effectively results in decreasing the dissipation losses in the filter and thereby increasing the effective transmission efiiciency thereof, and which amount of limitation prevents sustained oscillations in the system, in the absence of energy being fed into the input end of the filter.

A further object is to provide a two stage electronic tube amplifier which has positive feed back into the grid circuit of one or both of the stages, resulting in a greater amount of amplification, for the number of electrodes employed, than in prior art equivalent component organizations.

A further object is to provide one of the above said feed backs with a positive current proportional to the output load current of the amplifier, thereby compensating for output voltage drop, due to the applied load.

The teaching set forth herein, the structure characterized by the disclosure hereof and the methods recited in the claims appended hereto, may be employed in filters and amplifier organizations, other than and different from, that described in the specific embodiment of the invention exemplified in the disclosure thereof, Without departing from the spirit of the invention, as will be well understood by those skilled in the art. It is believed that the invention can be taught best, by one clearly defined. embodiment, rather than comprehensive generalities.

A good practical description of advanced prior art structure is given in A New Voice Frequency Telegraph System, Electrical Communication, vol. 10, No. 4.

The spirit of the present invention resides importantly in novel multiple use of single components in an organization thereof.

In the filter structure disclosed herein, a single metallic core high Q tank circuit is employed in low coupling relation to a low Q input circuit and to a low Q output circuit, and which low Q circuits may be air-core coils.

In a band-pass carrier current filter under intermittent operation, such as for instance telegraphic keying, the speed at which the keying modulations will pass through the filter, depends upon the time required by the components of the filter system, having energy storage properties, to be fully energized andthe time required for the reverse operation to take:place,,that is; the time required for these components to deenergize.

When a filter system is once energized and the input energy stopped, energy continues to flew out of the storage components toward both terminals of the filter until the stored energy isex hausted. the stored energy is exhausted;.theeminimum -to. which the stored energy is exhaustediimportantily determines the operational characteristics of the filter for keying or equivalent operation.-.

In the published prior art, great stress has been laid upon the time-required for the building up process in filters under" tn'insient input operation," butverylittle; if. anything: hasbeen stressed about" the equally important'stored energy exhaustion process:

All filter" operation theory which has come undernoticeby the applicant; is--b'ased upon "the theory that the energy-ifit'he filter'always travels ihonedi'rection-ata time and in one direction at a time only, and-that" directioniseither" from the input terminal impedance" toward the output terminal impedance; orfrom the outputtermin'al' impedance-toward the input terminal impedance.

U'nder"thisexplanation of operation it is shown that if the terminahimpedances 'are'pure' resist= ances or a value equa'l'to the so" called charac= teristic" impedance ofithe filter-network; that is, equalto ofthe network, the flow'of'energy;throughthe system is in" one direction: and"; in one. direction only: This is" admitted true for: filter networks operating, under steady state: conditions, but; it will he; shown" that".

is an incorrect value f.or..non-refl'ecti'on operation under input'energy, operation: where this energy is'interrupted; It will he shown. hereinafter that. for aj" filter network to. function under non-re,- fle'ctionpp'eration; the terminar'resi'stance should. he equalito butsatis'factory.operationtisrattained with other; neighhorhoodrvaluesw Qneaspect otthespiritofrthe invention-.residesr in. the "employment otamenergy. storage. system in the. filter. circuitshaving. aimin-imumtstorage. capacity, for vthe performance of: the filter-and with terminal impedances suited to exhaust-this. energy. in. an economicaLperiod of time...

Another important .aspect .of.. the spiritof -the filter organiation isl theesupplying of energy. to.-

the filter tocompensate for.-some.of.-the -filte dissipation losses, .to supplyypositive regenerative. voltage feedi-bacl'r to the; voltage. amplifier. andpo'sitive loadcurrent .eed'.-.baek.to the power am: plifier.

To moreclearly, disclose. the... invention. a

If the input energyis.-resumed1before?v l Referring to Fig. 1, I is a source of audiofrequency voltage, the components within the dotted area A represent the filter proper, 2 represents'a" hi'ghQtank having a reactor 3 and a condenser-1i; Reactor 3 has a high permeability compressed-.ddstircore of toroidal form with an outside diameterpf 1%" and a cross-section of /4" x ;:the -core' and the coil weigh 2 ounces; 5 andufi rarie cross-wound air core reactors having an outside diameter of 1 4', a depth of A2" and a weight. of 1%.ounces. Air core terminal sections are employed because the terminaliresistances are'fincluded" in the-coils. themselves, sincethe:output'ofithe'filter is taken acrosslthe terminal'fcon'd'enser. 1', thezinput" condenser being 8; therefbeing no" point in' using high Q high permeability" coils with. external'resistors in" series withth'em';

The input and. output circuits are coupled to the".high'Q"tan-"l circuit 2;,at pointsl'a and 10", with" a common coupling point I I.

With such" an arrangement; a, higher voltage is; obtained across'con'den'ser' T,",than..the drop across a pra'cticalterminal.resistance.

Condensers (standard ISUvoltirating, which is IOLtimesvoltage rating required)? 4; 1i. and i8. weigh 3;4'.ounces.' This makes an active material weight of 8.4 ounces, with 216 ounces for hardware. and mounting panel.

The organization vwithinthe dotted area: B is a two stage amplifier empl'oyinga twin triode. I2, having'ja voltage amplificatiomplate. circuit [3 with a grid inputicircuit l4,,connectedfacrossthe outputcondenser 1;. of filter A. I5'is power am plification circuitof amplifier B,',the gridthereof. having grid resistor l6. Grid resistor. It". has. a heavy section .I Tto carry; load current. from. out-. put transformer. l8. Power" amplification circuit [5 is'. coupled through. condenser l9. and "through fe'edhack resistor .2510 the output circuitioflfilter. A and to the input "circuit. l l'of "amplifier. B}.

Allresistorsand condensers. of amplifier. B;.pl1'1s components ISand 20, are arrangedaronndtwin triode I2; to conserve space,.i'nstead o'f-in. a. compartment under the. tube.v

With such a-joint associated; organizationas A and B,. somedissipation losses in. A. can. be. compensated forhy the..feed-.backenergyffrom B into A by means of. the feed backcircuit 19-211; This feed back energy inneutralizing. some oithe dissipation losses ofthefilter hasthe eiiectet raising the overall'Q of thefilter, whichrincreases the sharpnessof definition. oflth'el filter. cut-ofi characteristics.

the filter does not equaLthetotallfilterdissipation losses, the system .willl not sustain. oscillations after the removalof. input energy-from source. L.

Under normal. operation. the feed-backenergy to. the. filter. .islimitedLorstabilized: by. the grid. conductionin circuit] A. .The magnitude. of. feed.-

backtofilter. A, is. importantlyrdeterm-ined by resistor. 20,. and. the voltage... of bias 21 That. is; when the crest value of the feed-back voltage. equals the .bias. voltage,. the. grid..circuit.- becomes specific embodiment thereofwill'be given, as it conducting and the increased current in resistor Mai-sh.

This. allfmakes. for a. smaller, lighter filter. Solong as. the feedsbackenergy to- 20, caused thereby limits or stabilizes the feedback current. This grid conduction current also increases the plate current of circuit i3 and thereby increases the voltage amplification of B into the power amplification circuit.

The secondary load circuit of output transformer is is connected to the heavy section ii of grid resistor it at the point 23. When load is applied in load circuit 22, the voltage drop across heavy resistor ll, adds to the voltage of grid resistor it, thereby increasing the power amplification of amplifier B; this amplification is limited or stabilized by high resistor iii and the grid conduction current in the power amplifier circuit.

Thus both the voltage amplifier and the power amplifier have positive regenerative feed-backs with stabilization.

The coefficient of coupling between high Q tank circuit 2 and the filter input and output circuitscan be as low as of the order of l. or 2 percent, which makes sharp cutofi characteristics in the filter. The loss in transmission efficiency of the filter due to the very low coupling coefficient, is made up by the feed-back energy into the filter circuit, which maintains the sharply defined characteristics of the low coupling coefficient, and provides in effect the higher transmission efiiciency of a much larger coupling coefficient.

These properties all make for a smaller, lighter filter.

Fig. 2 shows two different steady state voltage transmission characteristics for a 630 cycle filter and amplifier of the structures above described. Curve C represents the amplifier transformer voltage with a constant filter input voltage for a range above and below the mid-band frequency, when the tank circuit and the input and output circuits are all tuned to 630 cycles. Curve D is for the same filter with the tank circuit tuned to 630 cycles, and the input and output circuits tuned to a lower frequency. Still lower frequency input and output circuit tuning brings the curve into more perfect symmetry about the mid-band frequency and further lowering of the input and output circuit tuning reverses the order of dissymmetry about the mid-band frequency.

Fig. 3 shows the overall voltage transmission characteristics for the same filter of Fig. 2, but under square wave modulation envelopes into the filter input circuit, such as illustrated by envelopes of Fig. 3. This modulation envelope is the most severe test for such a filter. Envelope F is a good oscillograph trace of the output transformer voltage for a square wave filter input modulation at common marking and spacing speeds, using the tuning employed to obtain curve D of Fig. 2.

The magnitude of amplifier output voltages under the operation illustrated in Fig. 3 for frequencies outside of the transmission band, that is, the spill-over for the adjacent channels is only about double the steady state values shown in Fig. 2, by curve D, that means that if the channels are spaced 180 cycles, the output circuit would have present about 2 percent voltage from the lower channel, and about 3 percent voltage from the higher channel. This performance is not equaled by any carrier-current telegraph filter of any size known to the applicant. The applicant does nothold that such performance is impossible with larger filters, but does hold that there is considerable diificulty and cost in attaining such values in prior art filters.

This difficulty is due to a great extent to the much larger amount --of stored energy present in multiple section filters, which has to be exhausted from the filter during each spacing interval. In the present invention, with its single smallstorage circuit, this difiiculty is greatly lessened, with considerable improvement of prior art structures.

Also in a single-core storage unit, less core material is required for the same inductance, for two reasons; first, the same core is used for all the turns of the reactance, and second, the inductance of a coil on a single core is proportional to the square of the number of total turns employed. That means, if four separate coils are used for a. filter mid-section, not only are four separate cores required, but also four times the total number of turns. For example, if N turns are required on a certain size single core to obtain a given inductance, this inductance is proportional to N If these N turns are equally distributed between 4 cores (as is common practice for a mid-section), the inductance of each coil will be, proportional to and of the 4 coils will be or, one fourth that of the single core coil, which single coil results in a higher Q (more sharply defined cutoff characteristics) and less stored energy (faster modulation frequency response under transient working) A further filter improvement can be made, if and when desired, under transient operation, by embodying the following discovery in filter terminal resistances, which may be included in the terminal reactors when indicated.

It is believed that this discovery can be taught by useof simpler filter circuits than that illustrated in B of Fig. 1, because the mathematical theory of such a circuit involves very complex and diflicult-tomanage algebra, and the disclosure desired to be made, can be taught very easily, by a very simple procedure.

Referring to Fig. 4, circuit G, there is shown a conventional single stage high-pass filter. Gonventional filter theory assumes the fiow energy in such a filter is always in one direction at a time, that is, it flows from left to right or from right to left. On this basis it is shown that if no reflections are to occur at the filter ends (which is merely another way of saying that the filter will not sustain oscillations of its own accord or that it is a non-oscillatory system), the terminal resistances must each equal the characteristic impedance of the system or While most treatments of the subject do not point out that the result is arrived at on a basis of the steady state analysis of the network, that is the case. Itis, of course, well known that this result has certain frequency limitations at tached to it, but it is the basis of a good working rulefor'steady state filter operation.

Referring again to Fig. 4, circuit G, if a modulated wave such as E, Fig. 3, is injected into one gasses terminal resistance of G, the LC? of the circuit has: to "fill 1111?: before the steady-state transmission voltage value shows up. at the other terminal resistance. of the filter; that is" illustrated by the form of the tracing of the envelope F, Fig. 3, during the crescent interval. While the curves in Fig. 3 are taken from the operation of the band-pass filter of Fig. 1, they can, notwithstanding, be, used as illustrating certain operations infilters having only high-pass or low-pass characteristics.

Ifithe filter G has its input-energy interrupted when the LC of the circuit is full of stored energy, the. flow of energy ceases to move in one direction; because of the removal, of opposing voltage at the input end, the stored energy; moves toward that end as well as toward the output end". This means that when the input voltage is removed from circuit G, after steady state operation is attained, circuit G1 operates exactly as circuit'H of the same figure, WhlCh'iS its exact equivalent.

Circuit H is the familiar closed circuit, system containing capacity, inductance and resistance in series, represented by'--the differentia1 equation:

It is well known that for a circuit represented by this equation to be non-oscillatory, R must have a value not less than that represented, by the following relation:

Thisidoublervalueuof B, when the energy is fiowingin, two. directions, provides the same resistance. facing, thedoublefiow of the stored energy thatthe single R provides for the unidirectional fiow of energy, because of the, resistance being in parallelrelation during double flow The exact same reasoning can-be. applied to a low-pass filter, asclearly indicated in Fig. 5, as well. as to bandpas s filters,

Thismeans that if, a. filter functions under 7 steady-state workinguin non-oscillatory operation with terminal resistances each equal to R; for. such, a filter to function undertransitorystate working in non-oscillatory operation, the terminal resistances must, be at least. equal to 2B.

Referring back to Fig. 1, this means, that, if andiwhen. desired, the non-oscillatory response of such acircuit can be materially improved by considerably increasing the resistance of the, terminal half-sections, over and above that resist,- ance which is, equal, to, the characteristic impedance of the network,

I claim;

1. In a two stage electric-wave narrow-band signal-amplifying system having a first stage including a first set of vacuum tube input and output amplifying elements and a tunedsignal circuit having attenuation loss connected to said input elements, a second set of vacuum tube input and output,amplifying elements, said second t ofinnut el m nts having a ridging resi tor across the control grid, and the cathode thereof, said first output elements being H connected to said second input elements said second output elements being connected 130 the primary wind. ing of a transformer having an isolated secondary winding having a loadgincluded in the circuit thereof, said second set of output elements having a positive feed-back circuit to said first set of input elements, the impedance of said feed-back circuit being sufiicient to prevent selfoscillation in said tuned signal circuit, and said isolated secondary winding having a positive feed-back circuit to an intermediate tap of said bridging resistor.

2. In a two stage electric-wave narrow-band signal-amplifying; system having a first stage including a first set;of vacuum tube input and'output amplifying elements and a tuned signal circuit having attenuation-loss connected'to said input elements, a second set of vacuum tube input and output amplifying elements, said second set of input elements havinga bridging resistor across the control grid and the cathode thereof, said first output elements beingconnectedto said second in put elements, saidsecond output elements being connected to the primary winding of a transformer having anisolated secondary winding having a load included in the circuit thereof, said second set of output elements having a positive feeca back circuit to said first set of input elements, the impedanceof said feed-back circuit being sufficientto prevent self-oscillation in said tuned signal circuit, said isolated secondary Winding having apositive feed-back circuit to an intermediate tap of said bridging resistor, and the grid voltage-drop of that portion of said resistor from said tap to the cathode end thereof being only a fraction of the total voltage 'of said secondary winding.

3. In a two stage electric-wave narrow-band signal-amplifying system having a first stage including a first set of vacuum tube input and output amplifying elements and a tuned signal circuit having attenuation loss connected to said input elements, a secondstage including a second set of vacuum tube input and output amplifying elements, said secondsetof input elements having a bridging resistor across the control grid and the cathode thereof, said first output elements being connected to said second input elements, said second output elements being connected to the primary winding of a transformer having an isolated secondary winding having a load included in the circuit thereof, said second set of output elements having a positive feed-back circuit to said first set oi input elements, the impedance of said feed-back'circuit being sufiicient to prevent self-oscillation in said tuned signal circuit, said isolated secondary winding having a, positive feed-back circuit to an intermediate tap of saidbridging resistor and the total grid voltage drop of said bridging resistor being less than that required to cause self-oscillation in said second stage.

N F RD MQR ISOR.

References Cited in the file of this patent UNI'I'EDSTATES PATENTS i945 OTHER-REFERENCES Terman text; Radio Engineering, 3d edition, pp. 323-326, published 19d? byMcGra-w Hill Co. New York city, N; Y. (Copy in Div. 69.) 

